IPSec Vulnerability Puts VPNs at Risk

A UK security group has warned that several attacks against IPSec could render encrypted transmissions as plaintext with “moderate effort.”

The National Infrastructure Security Co-ordination Center (NISCC) has issued a security alert for the vulnerability, which it describes as severe, saying that not only does it offer the potential to compromise secured communications, but that the attacks it has discovered “have been implemented and demonstrated to work under realistic conditions.”

According to NISCC, the vulnerabilities apply to certain IPSec (define) configurations that rely on Encapsulating Security Payload (ESP) in tunnel mode with confidentiality only, or with integrity protection offered by a higher layer (define) protocol.

The advisory describes the attacks as feasible through the interception and alteration of Internet Control Message Protocol (ICMP) (define) packets over an IPSec connection. Malicious users can observe segments of a packet header and access the inner payload in cleartext.

All three of the attacks are, according to NISCC, “probabilistic in nature and may need to be iterated many times in a first phase in
order to be successful.”

“Once this first phase is complete,” says the alert, “the results can be reused to efficiently recover the contents of further inner packets.” The report also notes that the attacks are fully automatable.

According to NISCC, there are three solutions to correct the vulnerability:

  • Configuring ESP to use both confidentiality and integrity protection, which it recommends most highly.

  • Using the AH protocol alongside ESP to provide integrity protection, which still poses risk to users where AH is in transport mode and tunneled inside ESP.

  • Filtering ICMP messages at a firewall or security gateway.

The full advisory is posted at http://www.niscc.gov.uk/niscc/docs/al-20050509-00386.html?lang=en

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